There are two main types of snail baits, and one is considered relatively safe for dogs. Look for the active ingredient and use the kind that contains 1% iron phosphate. This is relatively safe for dogs, because there is actually very little iron in the compound, and what there is, is poorly digested and absorbed by dogs, so most of it passes through them without incident. That being said, iron phosphate can still be toxic to dogs if they ingest enough of it: a 40 lb dog would have to consume about 3 lbs of this bait to receive a lethal dose of iron, although vomiting and diarrhea can occur with as little as about 1/10 of that amount.
Certain brands of snail bait contain an active ingredient called metaldehyde. Metaldehyde causes muscle tremors that progress to convulsions. Dogs can easily die from this poison. Each spring when the snails come out, we see numerous dogs at our practice that have ingested metaldehyde that require emergency treatment, including iv fluids, injections of anti-seizure medication, and a one or two-day hospital stay. Keep all potential poisons well out of reach of dogs. Pet-safe alternatives to snail baits with metaldehyde include Sluggo Slug and Snail Bait, handpicking, and copper barriers.